U.S. patent application Ser. No. 149,248, filed June 2, 1971, now U.S. Pat. No. 3,689,388, relates to polishing niobium parts electrolytically by a method in which the niobium part to be polished is placed as the anode in an electrolyte containing H.sub.2 SO.sub.4, HF and H.sub.2 O and a constant electric voltage is applied between the niobium part and a cathode, which is also disposed in the electrolyte. The constant electric voltage is adjusted so that damped current oscillations occur which are superimposed on the electrolytic current. Not later than after the complete decay of the current oscillations the voltage is turned off until the oxide layer built up during the current oscillations is dissolved. Subsequently, a constant voltage is again adjusted in such a manner that damped oscillations occur. The sequence of states with the voltage turned on and off is repeated several more times.
In the aforementioned method the niobium part to be polished is placed in an electrolyte consisting of 86 to 93% by weight of H.sub.2 SO.sub.4, 1.5 to 4.0% by weight of HF and 5.5 to 10% by weight of H.sub.2 O at a temperature between 15.degree. and 50.degree.C, and a constant voltage between 9 and 15 V is adjusted so that damped current oscillations occur.
During the current oscillations, which are superimposed on the electrolytic current and which decay after a time, excellent polishing action takes place at the niobium surface. At the same time an oxide layer is being built up, which leads to the decay of the oscillations. A deviation of about .+-.0.1 V is permissable from the adjusted constant voltage. With larger deviations oscillations that have sufficient amplitude no longer occur. By turning off the voltage the oxide layer is dissolved, so that current oscillations again become possible if the voltage is turned on once more. The voltage must be switched off no later than after the complete decay of the oscillations because otherwise the niobium surface to be polished is etched and thereby becomes undesirably rough.
By repeating the successive switching on and off of the voltage several times, mirror-smooth surfaces are obtained within a relatively short time. By repeating the switching sequence many times, relatively thick layers can be polished down without etching to achieve a final product with a mirror-smooth surface.
The optimum voltage for developing the current oscillations depends to some degree on the composition and the temperature of the electrolyte and can be determined experimentally by merely raising the voltage until the desired oscillations occur. It has been found particularly advantageous to work with an electrolyte containing 89.0 to 90.5% by weight of H.sub.2 SO.sub.4, 2.2 to 3.0% by weight of HF and the remainder H.sub.2 O at a temperature of 20.degree. to 35.degree.C, and with constant voltages between 11 and 13 V. Under these conditions particularly rapid oscillations occur which result in a particularly good polishing action.
It is not necessary to keep the voltage switched off until the current oscillations decay completely. In order to make the best use possible of the polishing action which occurs during the current oscillations, the voltage should be switched off, however, at the earliest only when the current oscillations have just passed their maximum amplitude. The earliest moment for switching the voltage on again can be easily determined experimentally in each particular case. As long as the oxide layer remains, no new current oscillations can occur when the voltage is switched on. In order to assure complete dissolution of the oxide layer built up during the current oscillations, the voltage should preferably remain off each time for about four minutes, with the electrolyte at rest. The dissolution of the oxide layer can be accelerated if one keeps the electrolyte in motion, at least at the surface to be polished of the niobium part. In that case, the voltage should remain off preferably for at least about 1.5 minutes each time.
The method according to the above-mentioned U.S. patent application has the advantage, among others, over other prior known methods for polishing niobium that otherwise difficult-to-control changes in the parameters of the polishing process, particularly concentration changes of the electrolyte, can be recognized by the change of the shape of the damped oscillations, for instance by the change of the oscillation frequency, the degree of damping and the magnitude of the maximum amplitude, during the condition with the voltage switched on. The parameter changes can therefore be immediately rectified at the next process step by changing the concentration ratio of the electrolyte components, or even more advantageously, by increasing the constant voltage to be adjusted in the range from 9 to 15 V.
The most important parameter change that occurs usually involves a slight reduction of the HF component in the electrolyte as the hydrofluoric acid is spent in dissolving the oxide layer present or being formed on the niobium part. This reduction of the HF concentration leads to a decrease of the maximum amplitude of the oscillations and can be compensated for by a slight increase in the voltage when the constant voltage is next switched on. The voltage increase required for the compensation of a reduction of the HF concentration by 0.25% is about 0.5 to 1 V. In a volume of electrolyte which is large in comparison with the size of the niobium part to be polished, the changes in the concentration of the electrolyte components are so small, if the duration of the process is not excessive, that corrections are not necessary. For the repeated switching on of the voltage the same constant voltage can always be adjusted in that case. The method is thereby simplified considerably.
The method according to the above-mentioned U.S. patent application is outstandingly well suited for the preparation of mirror-smooth niobium surfaces of high quality and for reducing entire surface layers with simultaneous polishing action. A high-quality surface is, for instance, required for superconducting cavity resonators of niobium, in which the superconductivity of the niobium is exploited. Such cavity resonators can, for instance, be used for particle accelerators. During the operation of the resonators high-frequency absorption takes place in the superconducting surface layer. In order to minimize the high-frequency absorption and losses, the surface layer should have a composition as homogeneous as possible, be as smooth as possible, and as free as possible of disturbances of any kind. The surface roughness, which is unavoidable in the machining of the niobium surfaces, must therefore be removed by polishing. As a rule it is furthermore necessary to remove a surface layer of the niobium part of several 100 .mu.m, to the extent that the former exhibits disturbances in the crystal lattice caused by the previous machining, which can cause losses. In general, mirror-smooth niobium surfaces are of advantage in all cases where high-frequency and/or a-c power losses in the superconducting niobium parts are to be avoided. This applies particularly to superconducting niobium separators for particle accelerators and to tubular niobium conductors for superconductive a-c cables.
While smaller niobium parts, particularly of geometrically simple shape, can be polished electrolytically in a simple manner by means of the aforementioned method without additional measures, it has been found, however, that considerable difficulties can be encountered in polishing the interior surfaces of hollow niobium bodies. In order to be able to polish electrolytically the interior surface of a hollow niobium body, the cathode must be introduced through an opening in the hollow niobium body into the interior thereof, with the niobium body itself connected as the anode. During the condition when the voltage is switched on, i.e., while current passes through the electrolyte between the cathode and the anode, a substantial development of gaseous hydrogen, which rises in the electrolyte, takes place at the cathode. This gaseous development has a very disturbing effect. First, there is danger, particularly in hollow niobium bodies of complex geometrical shape, for instance, in hollow niobium bodies which are to be used as separators in particle accelerators, that gas pockets form in the interior of the hollow niobium body, so that parts of the interior surface of the hollow niobium body to be polished are no longer wetted at all by the electrolyte and therefore cannot be polished.
It is, of course, possible under certain conditions, if the hollow niobium body is of sufficiently simple shape, to make provisions by a suitable disposition of the hollow niobium body and the cathode in the electrolyte, so that the gas bubbles formed at the cathode can rise to the surface of the electrolyte without the formation of gas pockets.
In a tubular hollow niobium body which is open on one or both sides, one can for instance, arrange a rod-shaped cathode in the axis of the tube and immerse the hollow niobium body with an opening pointing upward in the electrolyte in such a manner that the axis of the tube and the cathode are perpendicular to the surface of the electrolyte. By such an arrangement, however, the difficulties occurring due to the gas development cannot be reliably avoided. Particularly in hollow niobium bodies whose length is large as compared to the smallest inside dimension of its interior, gas bubbles which rise in the electrolyte penetrate practically the entire space between the interior surface of the hollow niobium body and the cathode, if the cathode is correspondingly long. The gas bubbles pass along the interior surface of the hollow body and come into contact with it. In such process the development of an anodic boundary layer, in which the voltage drop between the anode and the cathode is to take place predominantly, is obviously disturbed so much that the current oscillations required for good polishing action are largely suppressed, or do not occur at all. High quality surfaces can then no longer be achieved.